Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to obtain higher levels of pollution abatement and increased fuel economy. Among the known options, direct fuel injection appears to be one of the best candidates for achieving improved performance but higher initial costs have tended to discourage its general adoption. This situation is accentuated because only the more sophisticated and more expensive direct injection systems are capable of achieving the increasingly higher performance goals of engine manufacturers.
Until recently, attempts to provide a low cost fuel injection system have tended to center on distributor type fuel injection systems having a single centralized high pressure pump and a distributor valve for metering and timing fuel flow from the pump to each of a plurality of injection nozzles, such as disclosed in U.S. Pat. No. 3,557,765. Although simple in design concept, systems of this type generally suffer defects inherent with separation of the injector nozzles from the centralized pump. Unit injector systems avoid these inherent defects by providing each engine cylinder with its own cam-actuated pump such as disclosed in U.S. Pat. No. 3,544,008. Nevertheless, the performance advantage provided by unit injectors has generally not outweighed the detriment of the greater cost involved except when unit injectors are used for heavy duty compression ignition engine applications. The design of a commercially competitive unit fuel injector therefore normally requires the acceptance of some characteristics which are less than optimal, since the basic injector design goals of low cost, high performance and reliability are often in direct conflict.
As the need for higher engine efficiency and pollution abatement have increased, it has become increasingly evident that some economical means must be provided to vary injector timing in response to changing engine operating conditions. Such control is relatively straight forward in distributor-type fuel injector systems since the injection event is controlled at one central location. However, in unit injector systems, control over injector timing ordinarily requires modification of each individual unit injector, thereby adding significantly to the overall cost of the system.
Fuel injection systems for internal combustion engines that inject fuel into the engine cylinders at a non-uniform rate are well known. One method for varying the injection rate is disclosed in my U.S. Pat. No. 3,965,875, entitled "Fuel Injection System for Diesel Engines." In this patent, I disclose a fuel injector which injects fuel at a relatively slow rate during a first portion of the advancing stroke of the injector plunger and then at a faster rate during a second portion of the injector plunger stroke. During the initial portion of slow injection, an auxiliary coil spring absorbs some of the downward motion of an injector system rocker arm and slows the downward movement of the injector plunger.
U.S. Pat. No. 4,602,597 to Rhodes discloses a spring assisted hydraulic piston cylinder push rod system to vary automatically the valve timing of an internal combustion engine. However, in varying the valve timing, this system uses a restricted oil bleed, similar to conventional push rods having oil bleed holes, to hydraulically vary valve timing. Although this patent discloses a valve timing varying device mounted on a valve push rod, the device is highly complex, relying on a hydraulic, spring-assisted, piston cylinder arrangement that does not appear to be readily adaptable to use with fuel injectors.
A need exists for a simple variable rate fuel injector train that is easy to manufacture and package and that requires a minimal number of added parts to achieve a variable injection rate. Previous coil spring systems incorporating at least one coil spring strong enough to accommodate the loads required for an effective variable injection train spring rate, often include ancillary parts which must be machined to close tolerances. These systems also usually require modification of the plunger component for the fuel injector, and the added parts increase the mass of the injector drive train and require additional packaging.